The present invention relates to superconducting magnets used to generate a uniform magnetic field used in magnetic resonance diagnostics.
Present superconducting magnetic resonance magnets require cryogens to operate, either liquid helium or liquid helium and liquid nitrogen. Liquid helium is used in superconducting magnets not only for cooling but also to stabilize the magnet windings against motion induced instabilities. Cryogens evaporate and are lost during magnet operation and therefore require periodic cryogen delivery service and cryogen addition, with the attendant cryogen safety hazards. Furthermore, the use of cryogens complicate the cryostat construction since the cryogen containment vessels must be built in accordance with pressure vessel codes to withstand pressure surges as a result of magnet quenches or loss of vacuum in the vessel surrounding the cryogen containment vessel. The heavy cryostat containment vessel requires complicated supports and shields to position the cryostat containment vessel in the vacuum vessel and yet minimize heat conduction and radiation to the containment vessel from the ambient temperatures surrounding the vacuum vessel.
Helium leaks into the vacuum vessel surrounding the cryostat containment vessel are a common cause of failure in most superconducting magnets. Helium gas leaking into the vacuum vessel provides good heat conduction from the ambient temperature surrounding the vacuum vessel to the cryogen containment vessel containing the superconducting coils.
Cryogen costs, specifically helium, are increasing and there is a limited supply which is economically recoverable. In many parts of the world helium is not available, therefore magnetic resonance imaging systems based on niobium titanium (NbTi) superconductors cannot be operated.
There is a broad demand for special purpose magnetic resonance imaging systems in doctor's offices and remote locations to address medical diagnosis in extremities, mamography, temporomandibular joints and small organs.
It is an object of the present invention to provide a superconducting magnetic resonance imaging magnet which does not require the use of consumable cryogens.
Another object of the present invention is to provide a superconducting magnetic resonance imaging magnet which has a reduced size, cost, weight, and increased reliability compared to the same field strength magnet presently available.
A further object of the present invention is to provide a superconducting magnetic resonance imaging magnet which does not require a helium vessel or nitrogen cooled thermal shield.
A still further object of the present invention is to provide a superconducting magnetic resonance imaging magnet which has a smaller diameter cylindrical superconducting winding compared to presently available superconducting magnets with the same field, thereby reducing the amount of superconductor required and reducing the magnet stored energy which has to be dissipated in a quench.
It is yet another object of the present invention to provide a superconductive magnetic resonance imaging magnet in which the patient's field of view is wide open and claustrophobic fears are not aggravated.